In radio frequency (RF) applications of field effect transistors, in addition to a radio frequency signal, a DC network is connected to the gate and the drain electrode of a transistor providing a gate and a drain bias to set the operating point of the transistor.
A typical example of a field effect transistor with a gate bias network is shown in FIG. 1a and FIG. 2a. A gate electrode of a field effect transistor 1 is connected to a radio frequency source 3 and a DC or low frequency network 4. The DC network 4, here consisting of a DC source, sets the operating point of the transistor. As shown in FIG. 1, the radio frequency network 3 and the DC network 4 are usually coupled to the gate electrode of the field effect transistor via a bias-T device 6. The bias-T 6 feeds the DC to the transistor 1 and also decouples the RF input and output path. The bias-T usually consists of an RF choke (inductor) or RF block 13 in series at the DC path which acts as a through for DC, and a capacitor or DC block 14 in series at the RF path which blocks DC current to flow towards the RF source 3 but at the same time lets the RF pass through to the gate electrode of the transistor 1. The source of the transistor 1 is connected to ground. The transistor drain is also coupled via a drain bias-T 7 to a drain DC network 8, here just a DC source, and a load 2.
Under normal operating conditions, the transistor 1 the DC voltage at the gate is kept negative, for a High Electron Mobility Transistor (HEMT) usually between −5 to 0 V. A HEMT is a field effect transistor incorporating a junction between two materials with different band gaps, i.e. a heterojunction, as the channel. The amount of negative voltage of the gate bias network 4 controls the current flow in the channel to the drain. The negative voltage varies for different FET according to design and physical properties. Due to the negative voltage, a very small negative current flows from the DC network 4 to the gate electrode of transistor 1, indicated by the full black arrow in FIG. 2a. FIG. 2a is a schematic diagram of the same network as in FIG. 1 without the details of the bias-T's 6 and 7. A so called pinch-off condition of the DC network 4 is defined by the negative voltage when the channel is totally off, i.e. no DC current flows into the channel or to the load connected to the drain. An example of a radio frequency network 3 in FIG. 1 may be a power amplifier or an oscillator feedback circuit for establishing oscillations.
In power amplifier applications, the linearity specification is very important to prevent neighbouring channel interference. Linearity is, e.g., a crucial issue in modern wireless communication. Under normal driving conditions of the FET 1 in FIG. 1 and FIG. 2a, the RF signal 3 at the input is very small and the device 1 operates in its linear region, thus signal distortion is very small. In such a case, a very small negative current flow towards the gate of the device 1 and a high positive drain current flow towards the drain, see FIG. 2a. 
However, in communication front ends under overdrive conditions, the distortion components may disturb the wireless communication in a number of different ways. The amplifier may be overdriven by a very large input signal. As a consequence, the amplifier may produce distortion components, which interfere its neighbouring channel. The overdriving input signal may also result from an undesired origin. Possible undesired signals could be a large jamming signal from radar or any other transmitter. The large input signal may also come from the transmitter amplifier in a transceiver system. In FIG. 2b an overdrive condition of the device of FIG. 2a is shown, where the gate current of the FET 1 increases and becomes eventually positive as indicated by the arrow in the gate DC path. In this condition, the device 1 produces very high distortion products, which may damage the device.
In such overdrive applications, it would be desirable to adjust the operating point of the transistor or the DC source voltage to suppress distortion components.
Another very important application of field effect transistors in RF-engineering are oscillators. Here, not a signal from a power amplifier, but a signal from a feedback circuit may be connected to the gate electrode of a field effect transistor to generate periodical signals, the main task of an oscillator circuit. In most cases a sine in time domain is provided at the oscillators output. Square or triangle are also familiar wave shapes. Oscillators are available for a number of very different applications having very different properties. In communication applications, an oscillator with a very low phase noise is important. In case of large power generation, the noise in phase is less important.
In order to start and/or maintain the oscillation, a certain amplification of the signal is required. Therefore, the transistors bias point often destines the class of operation. Basically, a transistor may be operated in different classes. Several classes are known: A, AB, B, C, D, E, F. In class A, the linearity is very good, however the efficiency is very bad. It may theoretically up to 50%, but in most applications the class A efficiency is less than the ideal value of 50%. With classes AB, B, C, E, F the efficiency increases but the linearity decreases. Different classes of operation have different properties. For oscillator design, class A is very nice and easy to realize. In class C, the design becomes more difficult, because the start up condition is not fulfilled.
Therefore, also in oscillator applications of a field effect transistors, the setting of the bias point is important and adjustment in order to establish and maintain oscillations desirable.